Semiconductor devices typically include multiple layers of conductive, insulating, and semiconductive layers. Often, the desirable properties of such layers improve with the crystallinity of the layer. For example, the electron mobility and optical properties of semiconductive layers improves as the crystallinity of the layer increases. Similarly, the free electron concentration of conductive layers and the electron charge displacement and electron energy recoverability of insulative or dielectric films improves as the crystallinity of these layers increases.
For many years, attempts have been made to grow various monolithic thin films on a foreign substrate, such as silicon (Si). To achieve optimal characteristics of the various monolithic layers, however, a monocrystalline film of high crystalline quality is desired. Attempts have been made, for example, to grow various monocrystalline layers on a substrate such as germanium, silicon, and various insulators. These attempts have generally been unsuccessful because lattice mismatches between the host crystal and the grown crystal have caused the resulting layer of monocrystalline material to be of low crystalline quality.
If a large area thin film of high quality monocrystalline material was available at low cost, a variety of semiconductor devices could advantageously be fabricated in that film at a low cost compared to the cost of fabricating such devices beginning with a bulk wafer of semiconductor material or in an epitaxial film of such material on a bulk wafer of semiconductor material. In addition, if a thin film of high quality monocrystalline material could be realized beginning with a bulk wafer such as a silicon wafer, an integrated device structure could be achieved that took advantage of the best properties of both the silicon and the high quality monocrystalline material.
Accordingly, a need exists for a semiconductor structure that provides a high quality monocrystalline film or layer over another monocrystalline material and for a process for making such a structure.
This structure and process could have extensive applications. One such application of this structure and process involves the fabrication of electrical and optical devices from cubic GaN films. To simplify the following discussion, a reference to GaN is to be understood as including GaN, GaInN, AlGaN, SiN and AlN, unless the context makes it clear that only GaN is intended. GaN has a large, direct bandgap, structural stability and high thermal stability which makes it suitable for a wide range of electrical and optical device applications such as lasers, light emitting devices in the blue and green wavelengths, high temperature devices and solar blind detectors.
One significant challenge to large scale production of cubic GaN devices is the lack of bulk substrates formed of suitable lattice-matched material for subsequent high quality epitaxial GaN growth. Currently, GaN film growth is carried out on sapphire substrates or SiC substrates, both substrates of which present disadvantages. SiC substrates are of small size and are expensive. Further, GaN on sapphire is hexagonal and exhibits lower mobility than cubic GaN. In addition, sapphire has a lattice constant and thermal conductivity significantly different from III-V nitrides such as GaN and is electrically insulating. For example, the lattice constant for GaN differs by approximately 13–16% from that of sapphire. These significant differences lead to mechanical stresses in the subsequent film growth above the critical thickness, which results in fracturing and voids in the GaN layer. Another disadvantage of typical GaN films grown on sapphire is the high number of defect dislocations in the GaN layers. These defects impact the electrical and optical performance of the GaN devices. For example, in optical devices, the defects act as scattering centers requiring a higher laser threshold current density. In electrical devices, dislocations can create deep defect energy levels that increase the leakage current.
Accordingly, a need exists for a semiconductor structure that provides high quality electrical and optical devices formed of GaN films and for a process for making such a structure.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.